In a wireless voice/data communication system, a base station is associated with a sector. Mobile stations within the sector communicate with others, or with other networks via the base station. Typically, each mobile station only communicates with a single base station, so data will be freely transmitted from the mobile station to the base station. The base station, however, must be able to communicate with all of the mobile stations within its sector and must therefore be able to schedule transmissions to the various mobile stations.
One type of scheduler that may be used by the base station is referred to as a proportional fair scheduler. A proportional fair scheduler is designed to balance fairness of service among the mobile stations against the maximization of overall throughput of data by the base station. The scheduler compares the ratio of the requested data rate to the average throughput of the different mobile stations to take scheduling decisions.
The rate at which data can be transmitted to a given mobile station is determined by the signal-to-interference-and-noise ratio (SINR) at which the signal is received. With higher SINRs, data can be more reliably transmitted, so higher data rates are possible. With lower SINRs, transmissions are less reliable, so lower data rates are used. Since mobile stations' signal-to-interference-and-noise ratios generally fluctuate, the data rates also fluctuate.
The effect of using the proportional fair scheduler is to schedule users when their SINRs are large with respect to their average values, increasing the overall throughput of the base station while achieving fairness among users. As a result of the SINR fluctuations, the scheduler tends to distribute service among the mobile stations, each at its peak data rate.
By riding the peaks of the mobile stations, the scheduler achieves what is referred to as multi-user diversity gain. Multi-user diversity gains generally increases when the number of mobile stations in the sector increases, and when the dynamic range of fluctuation in the SINRs of the mobile stations increase (even when the average SINRs may remain the same).
While it may not be possible to increase the number of mobile stations in the sector, there are ways to increase the SINR fluctuations. One way to increase SINR fluctuations is to use a forward power control scheme that induces fluctuations. For example, in one such scheme, a beam which is formed by multiple, phase-shifted antennas is rotated to sweep through the sector. As the beam sweeps across a particular mobile station, the SINR for that mobile station peaks and then falls off.
This forward power control scheme works well if there are a relatively large number of mobile stations distributed throughout the sector, as one of them will usually be peaking at any given time and will therefore be selected by the scheduler. If, however, there are relatively few mobile stations in the sector, this scheme does not work as well. This is because there are periods when the beam is not directed to any of the mobile stations, so they all have relatively low SINRs and low data rates. Thus, when the sector has few active users the multi user diversity gain is low and the overall performance is degraded by the induced SINR fluctuations.